1. Field of the Invention
This invention relates to an improved matrix keyboard involving multiple electrical measurements per key coordinate. More specifically, the invention relates to a matrix keyboard involving a plurality of electrical measurement outputs to establish column and row coordinates of the key being pressed such as to increase the total number of keys on the keyboard.
2. Description of the Prior Art
Multikey keyboards for inputting data into a computer generally involve a plurality of normally open switches wherein the pressing of each key of the keyboard closes a corresponding switch. The set of normally open switches can be viewed as defining a switch matrix or switch array which in the conventional prior art keyboards have a plurality of input lines and a plurality of output lines. Pressing a key on the keyboard thus closing the corresponding switch creates electrical continuity and completes the circuit between the specific input and output lines corresponding to the respective position or identity of the key being pressed. This plurality of keyboard leads, in the conventional keyboard, is connected to at least one encoder which translates or converts the signals from the multiple key input circuits into a single alphanumeric coded output signal; e.g. into ASCII or binary coded decimal (BCD) format or the equivalent. The output signal from the keyboard encoder can then either be directed to the computer as the keyboard input or can be further processed by additional dedicated keyboard circuitry, depending on the particular manufacturing and overall computer system requirements, before entering the computer. Thus, it is not uncommon to find that the keyboard in addition to having an encoder will have its own dedicated memory and logic circuit essentially duplicating, in principle, that which the central processing unit (CPU) is theoretically capable of doing and consequently, increasing the cost of the keyboard. In practice the presence of multiple input and output lines from the keyboard to the encoder tends to limit the number of keys present on the keyboard.
In a recent copending U. S. patent application based on and claiming priority to a French application, the present inventor discloses a resistive multikey computer keyboard involving an inexpensive yet reliable printed circuit array of flexible keys. According to this matrix keyboard, a portion of a series of sequential resistors are brought into the keyboard output circuit such that the cumulative values of the output resistance circuits uniquely determines the X and Y ranks of the pressed key. In a pair of concurrently filed and commonly assigned U.S. patent applications, multikey keyboards based on time delay elements and capacity circuit values are disclosed. In principle, the number of keys present on a matrix keyboard based on any arbitrary electrical measurement of a cascaded serial sequence of electrical elements selectively introduced into the keyboard output circuits is limited by the ability of the computer to resolve and distinguish successive cumulative electrical measurement values. The present invention is considered an improvement on the basic concept of the cascaded sequential electrical element keyboard in that it provides a method of exponentially increasing the number of keys present on a matrix keyboard without exceeding a given sensitivity and dynamic range of the existing computer's measurement capabilities.